Simplify the following expression: $k = \dfrac{n}{5} - \dfrac{n}{4}$
Solution: In order to subtract expressions, they must have a common denominator. The smallest common denominator is the least common multiple of $5$ and $4$ $\lcm(5, 4) = 20$ $ k = \dfrac{4}{4} \cdot \dfrac{n}{5} - \dfrac{5}{5} \cdot \dfrac{n}{4} $ $k = \dfrac{4n}{20} - \dfrac{5n}{20}$ $k = \dfrac{4n -5n}{20}$ $k = \dfrac{-n}{20}$